In general, a clothes processing apparatus having a drying function, such as a washing machine or a clothes dryer, is an apparatus in which clothes (laundry, etc.), which have completely washed and then dehydrated, are introduced into a drum and hot air is supplied into the drum to evaporate moisture contained in the clothes so as to dry the clothes.
As one example, a clothes dryer includes a drum rotatably installed inside a main body for accommodating clothes therein, a driving motor for driving the drum, a ventilation fan for blowing air into the drum, and a heating unit for heating up air introduced into the drum. The heating unit may be a type of heater, which uses electric resistance heat of high temperature generated by electrical resistance or heat of combustion generated by burning gas.
Meanwhile, air exhausted from the drum becomes hot humid air due to containing moisture from the clothes within the drum. Here, depending on a method of treating such hot humid air, dryers may be classified, for example, into a circulating type dryer, in which hot humid air circulates without being exhausted outside the dryer to cool air below dew-point temperature through a heat exchanger, and accordingly moisture contained in the hot humid air is condensed to be resupplied, and an exhausting type dryer, in which hot humid air passing through the drum is exhausted directly to the outside.
For the circulating type dryer, in order to condense air discharged out of the drum, air should be cooled below the dew-point temperature and heated by the heating unit prior to being resupplied into the drum. Therefore, when a heater is employed as the heating unit, a heat exchanger for condensing hot humid air discharged out of the drum is separately needed, and thereby thermal energy supplied by the heater is rather discharged to the outside due to heat exchange with the heat exchanger. This structure has an advantage of supplying thermal energy as much as being necessary by use of the heater but causes a problem of lowering thermal efficiency and increasing energy consumption. Also, for air circulation, moisture should be removed sufficiently, which causes the heat exchanger to be larger in volume and a drying time to be longer.
For the exhausting type dryer, hot humid air should be exhausted to the outside and external air of room temperature should be introduced and heated up to temperature as high as being required by means of a heating unit. When a heater is employed as a heating unit in the exhausting type dryer, it has advantages of not separately needing a heat exchanger and reducing a drying time by supplying thermal energy as much as being required by use of the heater. However, the hot air discharged to the outside is discharged directly to the outside with containing thermal energy transferred by the heating unit, thereby lowering thermal efficiency and increasing energy consumption.
Therefore, in recent time, a dryer, which is configured such that non-used energy is recollected from air discharged out of the drum and reused for heating air supplied into the drum so as to enhance energy efficiency, is being introduced. An example of such dryer is a dryer having a heat pump system. The heat pump system includes two heat exchangers, a compressor and an expander (expansion apparatus). With the configuration of the heat pump system, a refrigerant which circulates in the system recollects energy contained in hot air exhausted and the recollected energy is used to heat air supplied into the drum, thereby enhancing the energy efficiency.
In detail, the heat pump system includes an evaporator at an exhaust side from the drum and a condenser at an inlet side of the drum. Accordingly, a refrigerant absorbs thermal energy through the evaporator and then is pressurized by a compressor to be a refrigerant of high temperature and high pressure. Afterwards, the thermal energy contained in the refrigerant is transferred to air, which is introduced into the drum, through the condenser, which allows hot air to be generated by using the energy which is wasted.
However, when the heat pump system is applied to a dryer, a performance of the system depends on capacities of an evaporator for absorbing thermal energy and a condenser for discharging energy, and a capacity of a compressor for compressing a refrigerant. Hence, it is ideal to design the heat pump system based on required thermal energy, but sizes or capacities of the compressor used in the dryer, and the condenser and the evaporator serving as heat-exchangers, actually have to be limited. Therefore, when the heat pump system is used as a heating unit for heating air supplied into the drum, it has an advantage in the aspect of energy efficiency but has a problem of increasing a drying time due to the capacity limitation of the heat pump system. In addition, regarding the characteristic of the heat pump system, an overload may occur in the compressor the like, resulting in lowered reliability of the heat pump system.
Upon performing a drying operation merely using the heat pump system, a quantity of heat transferred from the condenser to air is insufficient and accordingly a drying time is extended. To overcome such problem, a dryer having an auxiliary heater in addition to the condenser heat pump system has been introduced. Accordingly, when heat is not sufficiently supplied from the condenser due to the heat pump system not reaching a normal state at the beginning of running thereof, the auxiliary heater runs together with the heater to supply hot air heated up to temperature appropriate for drying from the initial operation, and also more heat is supplied by the heater even when the heat pump system is in the normal state, thereby reducing the drying time.
Here, in the circulating type dryer, since air circulates via the evaporator and the condenser, the heat pump system can normally operate only when temperature and humidity of air when flowing through the evaporator and the condenser are within an appropriate range. That is, when the evaporator fails to sufficiently cool hot air introduced into the evaporator due to excessively high temperature of the hot air, air having temperature over an appropriate range is introduced into the condenser, thereby lowering a heat transfer performance of the condenser. Consequently, when the refrigerant flowing through the condenser is not cooled sufficiently, a liquid refrigerant is introduced into the compressor or discharge pressure of the compressor is excessively raised, thereby badly influencing on reliability of the compressor.
If an auxiliary heater is added to the circulating type dryer having the heat pump system, the temperature of air introduced into the evaporator increases much more, which makes it more difficult to prevent an excessive load from working on the compressor at the beginning of running the heat pump system.